WASHINGTON -- Former Maryland Gov. Martin O'Malley said Friday that he would expand Social Security benefits and lift the payroll tax cap on the wealthy if elected president, as progressives wait to see whether the candidate he's trying to catch up to, former Secretary of State Hillary Clinton, will offer a similarly ambitious proposal.

O'Malley's plan comes as Democrats are mounting a more and more aggressive defense of Social Security in the face of conservative efforts to raise the retirement age and partially privatize the program, among other measures. Nearly every member of the Senate Democratic caucus offered a largely symbolic budget amendment in March to expand benefits, in an indication of just how polarized the two major political parties are on the issue.

Social Security is only projected to be able to pay all promised benefits until 2034, after which it will be able to pay approximately 75 percent of those benefits. Nearly two-thirds of seniors rely on the program’s benefits for most of their income, and the same is true for an even higher percentage of former workers with disabilities.

Republicans have used this long-term reality to call for radical changes to the program, while Democrats have instead proposed raising the cap on the payroll tax beyond the current limit of $118,500. Legislation introduced by Sen. Bernie Sanders (I-Vt.), who is also running for president as a Democrat, would raise the cap on the payroll tax to $250,000, to "make the wealthiest Americans pay their fair share."

Clinton is now the only one of the three major candidates seeking the Democratic presidential nomination who has not specifically addressed how she'd approach the program. But earlier this month, in a sign of how the dynamics of the issue within the party have shifted, she signaled at a town hall in New Hampshire that she'd be open to raising the cap.

"We do have to look at the cap, and we have to figure out whether we raise it or whether we raise it a little and then jump over and raise it more higher up," Clinton said then.

O’Malley’s plan would allow for up to five years of “caregiver credits” for those who take time out of the workforce to take care of children or elderly parents. Clinton hinted that she supported this sort of proposal when she told the AFL-CIO last month that she wanted to "enhance" Social Security by improving the benefits available to women. She's presumably considering a provision like O'Malley's, since one of her policy advisers, Ann O'Leary, wrote a Center for American Progress report in 2012 that focused in part on credits for caregiving.

Her comments on Social Security in this election cycle are notably different from her remarks on the issue during the 2008 presidential race, when she criticized then-Sen. Barack Obama for offering a proposal similar to Sanders' in which income over $250,000 would be subject to the payroll tax.

At the time, Clinton said that she was "certainly against" Obama's plan, because she didn't "want to raise taxes on anybody." She said she'd set up a bipartisan commission as president to address the program's solvency.

To that end, the Progressive Change Campaign Committee sent an email to its members Friday calling Social Security expansion "a winning issue" for Democrats. The group asked its members to sign a petition telling "ALL" Democratic presidential candidates to back the proposal.

Clinton told the AFL-CIO that she'd offer her Social Security plan "in the weeks and months ahead." Her campaign has pointed out on several occasions that Clinton has long opposed privatization proposals and cuts to benefits.

O'Malley's policy proposal would aim to increase benefits for retirees by lifting the payroll tax on income above $250,000. It would also increase the minimum Social Security benefit to 125 percent of the poverty line for people who have worked for 30 years or longer, and it would use a different cost-of-living index to more accurately reflect the higher costs of living that retirees now face.

In a Friday op-ed for Iowa's Quad-City Times, O'Malley rejected what he called the "sky is falling" approach favored by those who want to alter the program.

"Let’s be clear: Social Security is not in crisis. It is not bankrupt, and it would not be better off if it were privatized and run by Wall Street. Those claims are nothing more than misguided attempts to score political points at retirees’ expense," he wrote. "It’s not the sky that’s falling. It’s the floor that’s falling out from under our seniors."